This invention relates to porous glass bodies and to a method of storing volatile substances in such bodies so as to permit a regulated evolution of such substances.
In industry, in the household and in hospitals, in agriculture and forestry and in the interior of vehicles, there is wide use of devices which emit gaseous fragrances or active compounds serving, for example, as air fresheners, insecticides or insect-attractants (by release of pheromones).
All of these fragrance or active-compound emitters have in common that by evaporation or sublimation, they are supposed to release the fragrances or active compounds over a prolonged period of time as uniformly as possible with the concentration remaining the same and the composition being unaltered. The period of time over which the fragrances or active compounds are to be released is, in the case of fragrances for air freshening purposes, generally about 4-8 weeks, and in the case of active compounds such as insecticides it may be a number of months.
Carrier media used heretofore have considerable disadvantages including the fact that the release of the stored substance is often too fast or too slow. For example, a widely used device utilizes a cellulose membrane which closes off a container containing the fragrance. Such membranes have no defined pore structure so that the release of material can be controlled only with difficulty. In the case of fluctuating air movements, the membrane can dry out very quickly; the supply of stored material then stagnates. Furthermore, an aged membrane has a selective action so that the entire range of active compounds is no longer liberated, i.e. the composition of the active compounds changes with time.
A simpler way of storing and releasing fragrances and active compounds is the use of porous bodies in which the fragrance or active compound is located in the pores of the porous body and is vaporized therefrom.
Porous ceramic bodies, for example those produced by Rauschert Verfahrenstcelnick of Steinwiesen, Germany, in general, have relatively low total pore volumes of up to about 50% of the sintered body, with the volume of open pores being up to about 44% of the sintered body. Thus, the materials to be vaporized can be stored internally to only a limited extent so that only relatively small amounts of vaporizable material can be released per unit volume. Another disadvantage of ceramics is that the mineral materials unavoidably present in ceramics can lead to the selective fixing and thus separation of scents.
Other inexpensive materials which are frequently used as carriers are entire bodies of cellulose fibers, in particular cellulose felts or nonwovens. These bodies have no defined pore structure and, as a result of their fibrous structures, reduce the action of the capillary forces. The numerous hydroxyl groups of cellulose, by virtue of their interaction with polar groups of the fragrances or active compounds and with water molecules, lead to an undesired and nonuniformly strong fixing of active compounds and scents which can occur even during storage, as well as to undesired chromatographic effects occurring during the release of active compounds. Owing to these phenomena, especially in the case of highly active compounds and scents or mixtures of materials, the desired consistency of performance is impaired, for example, shifts in the fragrance in comparison with the non-adsorbed original material.
A further disadvantage of the cellulose bodies is their swellability. Under relatively high atmospheric humidity, the cellulose body can break up in layers and the excessive access of air which follows reduces the time of evolution and alters the desired concentration of active compound in the air of the room. In use, the cellulose bodies can also suffer deformation and discoloration, which adversely affects the aesthetic and sales values of products using these carriers.